Capitella capitata represents a complex (Grassle & Grassle, 1976) of over ten sibling species (Gamenick & Giere, 1997). While the species of this complex show only slight differences in adult morphology, they differ clearly in ontogenetic, ecological and genetic features (Gamenick & Giere, 1997) and have distinct reproductive modes (Grassle & Grassle, 1976).

In non-polluted areas, densities can exceed 250,000 ind/m² (Barnes, 1994). In polluted sites in Barcelona, densities of 440 000 ind/m² (Mendez et al., 1997) and 750 000 ind/m² (Sarda et al. ,1995) have been recorded. Tsutsumi (1990) recorded a maximum density of 200 000 ind/m² in sediment enriched with the green alga Ulva pertusa. Mendez et al. (1997) suggested that Capitella capitata is able to produce many individuals when organic supply is high enough to feed all the population.

Petraitis (1991) studied sex ratios in Capitella capitata (species type 1) and found that, although large populations contain males, females and occasionally hermaphrodites, at low density or in groups with a females-biased sex ratio males develop into hermaphrodites. Petraitis (1991) also noted that most homogametic juveniles become females if reared alone, but males if reared with other conspecifics.

Lopes et al. (2000) found that an increase in abundance of macroalgae resulted in a substantial increase in the abundance of Capitella capitata. Furthermore, Qian & Chia (1991) found that, in Canada, individuals fed on the bull kelp Nereocystis luetkeana or the green alga Ulva lactuca grew much faster and attained a larger body size than those fed on the kelp Macrocystis integrifolia or seagrass Zostera marina.

Qian & Chia (1991b) found that larvae grew faster and had shorter pelagic periods when fed with phytoplankton at concentrations of 10x and 25x ambient concentrations.

Age at maturity
Studies on natural populations in England show that sexual maturity is reached at about 4 months (Warren, 1976). However, in other geographical locations, sexual maturity may be reached at 3.5 months (Qian & Chia, 1994). In the laboratory, sexual maturity may be reached between 31 and 48 days after recruitment, depending on temperature (12.6-22 °C) (Tsutsumi & Kikuchi, 1984).

Reproduction

Warren (1976) noted that spawning occurred throughout the year in Plymouth, with all oocytes being released at a single spawning. Warren (1976) also noted that oocytes are not released into the coelomic fluid until almost fully developed and that larval development may have been completely benthonic. However, in the USA another variant of Capitella capitata, Capitella species 1, has been shown to have planktonic larval development for a short time (hours to days) before settlement (Grassle & Grassle, 1974). This species is considered to be iteroparous, and the lecithotrophic larvae are brooded during part of their development within the adult tube.

Holte & Oug (1996) noted that in northern Norway, Capitella capitata occurred in two variants; one small form with large eggs (250 µm) and one large form with small eggs (100-125 µm). Similar variants have been found in British waters (Pearson & Pearson, 1991).

Larval development

Planas & Mora (1989) calculated that individuals from the north west of Spain take 2-4 weeks to change from eggs to the juvenile stage and about 3 months from juveniles to adults.

Cultures of sibling species have indicated a generation time of 30-40 days (Grassle & Grassle, 1976; Grassle, 1984; Whitlatch & Zajac, 1985).

Capitella species 1 larvae were attracted by a sulphide concentration of 0.1mm to 1.0mm, yielding higher settlement, subsequent metamorphosis and survival of settled polychaetes compared with non-sulphidic controls (Cuomo, 1985).

Fecundity
Fecundity has been recorded as 460 eggs per female in Barcelona (Mendez, 1995) to 6-600 eggs per female in USA (Grassle & Grassle, 1974).

Longevity
Potential longevity ranged from 45 days in Barcelona (Mendez et al., 1997) to 2 years in a population in England (Warren, 1976).

Other information
Lopes et al. (2000) found that animals fed on the sea grass Zostera marina had the smallest body size, became sexually mature in the shortest period and had the highest average fecundity. Number of eggs produced from the first spawn was the highest for the individuals fed on the bull kelp Nereocystis luetkeana and the lowest for individuals fed on Zostera marina. Animals fed the green alga Ulva lactuca and the sea grass Zostera marina produced much larger eggs.